How to automatically join stripped pieces of video?
Keep the cursor (the green vertical line) on the left hand side, then ShiftBackspace to remove all gaps. (If pressing Backspace once, it will only remove the first gap to the right of the current frame, which can be operate continously. This operation can be found at Strip > Remove Gaps)



P.S.:

Here, Gap means frame durations that don't hold any strips. So, in case of multiple channels, you may want to first select strips on particular channel, then CtrlG to make them as a temporary meta strip, then Tab into that Meta strip, do what you want to do. Then Tab again to exit meta group, AltG to ungroup it.